The present invention relates to producing hydrogen (bi)sulfide with microorganisms. The hydrogen (bi)sulfide may be used for removal of metals from a waste stream.
As used herein, the term xe2x80x9chydrogen (bi)sulfidexe2x80x9d is defined as hydrogen sulfide and/or hydrogen bisulfide.
Industrial (e.g. metal finishing and electronics industries) metal-laden waste streams and environmental restoration activities (e.g., mine drainage) are the focus for treatment. Treatment of these waste streams represents a significant economic cost. The current baseline treatment is conventional pH neutralization followed by precipitation of the metal as sludge, such as by the addition of lime or sodium hydroxide to neutralize the waste and precipitate the metal(s). The conventional process produces large volume of difficult-to-handle, hydroxide sludge that must be disposed of or blended with other material to make it suitable for smelting (reuse). Other technologies can be applied to remove metals from waste streams and produce more manageable sludges/secondary wastes, but these technologies are typically more expensive than the conventional process due to higher energy consumption or higher reagent cost. (e.g., reverse osmosis, ion exchange, chemical sulfide precipitation). For example, U.S. Pat. No. 5,338,460 describes removing dissolved heavy metals from an aqueous stream by reacting at least one of the metals with a water-soluble inorganic sulfide at a controlled pH between about 2 and 3.5 within a temperature range of between 100 degrees F. and 212 degrees F. However, this process has not been commercially accepted because chemical sources of sulfides for use in precipitation processes (e.g., sodium sulfide) are expensive such that chemical sulfide precipitation costs are typically higher than costs for baseline technologies.
The use of sulfate-reducing bacteria (SRB) has been proposed as a cost effective means to remove metals from waste streams. In fact, there are several successful applications of this process in operation for environmental restoration efforts (mine drainage treatment and groundwater treatment) de Vegt, A. L., H. Dijkman, and C. J. Buisman. 1998. xe2x80x9cHydrogen Sulfide Produced from Sulfate by Biological Reduction for use in Metallurgical Operations.xe2x80x9d In: Proceedings of the Society for Mining, Metallurgy, and Exploration Annual Meeting, Orlando Fla., Mar. 9-11, 1998. SRB can be used to remove metals for waste streams because these bacteria produce sulfides (hydrogen sulfide (H2S) and bisulfide (HSxe2x80x94)), that react with metals to form insoluble metal precipitates. Thus, systems can be designed that induce sulfide generation by the SRB and then contact the resulting sulfide with metal-laden waste streams such that the metals precipitate from solution and are removed from the waste stream.
SRB can be found in nature in a number of environments in which some portion is anaerobic (e.g., municipal sewage sludge, river and marine sediments, aquifers). SRB typically live in conjunction with other bacteria that convert complex organic molecules into the more simple organic molecules and hydrogen that SRB can metabolize. SRB link oxidation of an organic or hydrogen to reduction of sulfate (and in some cases other oxidized sulfur compounds) to produce sulfides as a product. During metabolism, some SRB can completely oxidize small organic acids to CO2 and water. Other SRB incompletely oxidize organic acids such as lactate to acetate. SRB can survive some exposure to oxygen, but are strict anaerobes and only reduce sulfate in the absence of oxygen. Sulfate is the primary electron acceptor for SRB, although, some species/strains can reduce other compounds (Widdel, F. 1988. xe2x80x9cMicrobiology and Ecology of Sulfate- and Sulfur-Reducing Bactieria.xe2x80x9d In: A. J. B Zehnder (Ed.). Biology of Anaerobic Microorganisms. Wiley Interscience, New York).
As is taught by U.S. Pat. No. 5,587,079, sulfides produced by SRB may be used for metal sulfide precipitation. The reaction chemistry of the precipitation process is:
M2++S2xe2x88x92xe2x86x92MS
where M2+ represents a metal ion having a valence of 2+,
S2xe2x88x92 represents a sulfide ion with a valence of 2xe2x88x92, and
MS represents a metal sulfide compound (solid precipitate).
Bacteria are used to provide hydrogen sulfide and carbonate compounds for treating solutions containing metal ions. By controlling the addition of hydrogen sulfide and carbonate compounds to the solution, the preferential isolation of particular metal sulfide concentrates may be accomplished in separate precipitation steps, each with a specific pH and sulfide dosage.
U.S. Pat. No. 5,554,290 reports use of microbially-generated (biogenic) sulfide for metals precipitation for in situ environmental restoration. In this method, nutrients that stimulate indigenous bacteria to produce sulfide are injected into the subsurface aquifer. The sulfide precipitates metals present and the precipitate remains in situ.
In U.S. Pat. No. 4,735,723, waste water containing sulfate and organic material is purified by anaerobic biological waste water treatment where at least 80% of the sulfate is converted into hydrogen sulfide in an acidification process and at least 70% of the resulting hydrogen sulfide is removed from the waste stream.
However, although the use of SRBs produce sulfides useful in the removal of heavy metals from process streams, such processes currently known in the art are essentially steady-state processes that have not heretofore been used for xe2x80x9con-demandxe2x80x9d production of sulfides for metal removal. Such limitations are a result of the fact that the sulfide production rate of a biological reactor is proportional to the concentration of viable bacteria in the reactor. That is, the overall production rate is the production rate of a single bacterium (or a specific production rate per gram of bacteria) multiplied times the number of bacteria (or total grams) in the reactor. The concentration of bacteria in the reactor is herein after termed the biomass concentration. A material balance for biomass concentration in a reactor is as follows.
Mass rate of change in biomass=rate of change due to growthxe2x88x92rate of change due to decayxe2x88x92rate of change due to outflow.
The rate of change due to growth represents the bacterial growth from metabolism of a substrate, i.e., substrate conversion. The rate of change due to decay is the decrease in biomass concentration due to maintenance energy requirements and endogenous metabolism which is commonly related to the use of cellular materials or extra-cellular materials as a substrate. In a reactor at steady-state operation, the bacterial growth is balanced by the loss of bacteria in the reactor effluent and the rate of bacterial decay. At this steady state condition, the sulfide production rate of the reactor is stable.
For reliable xe2x80x9con-demandxe2x80x9d sulfide production, a reactor must be shut down and then restarted at the same conversion rate (i.e., at the same steady state condition). Therefore, the biomass concentration must be maintained at the same concentration during the time intervals between active demand for sulfide. However, currently, when the inflow and outflow to the reactor are stopped and substrate is not added to the reactor, over time, the biomass decay rate will predominate such that the biomass concentration decreases. To restart the reactor thus requires either adding a charge of new bacteria to return the reactor to the previous level of biomass concentration, or allowing sufficient time for the remaining bacteria to multiply to the previous level of biomass concentration.
Hence, for reliable xe2x80x9con-demandxe2x80x9d sulfide production, there remains a need for a method to control the process of microbial production of sulfides wherein the rate of sulfide production may be first halted, and then resumed, without the necessity of adding additional bacteria or waiting for the remaining bacteria to multiply and increase the biomass concentration to the prior level. The need additionally extends to match sulfide production with demand, for example metal precipitation.
The present invention provides a robust and stable means to stop and start sulfide (H2S) production in a biological system, without diminishing the longterm performance of the biological system, such that the biological system operates as if it were a container of H2S gas with a control valve.
Advantages of the present invention include 1) coupling microbial production to the use of H2S [ H2S is toxic and thus preferably not produced in excess of demand], 2) stopping the production of sulfide when it is not needed for metal precipitation, thereby conserving consumables, reducing costs, and increasing safety, 3) stopping and then re-starting the sulfide production without impacting the efficiency of the production mechanism (bioreactor).
The critical feature of the present invention is the careful control and rapid changes of pH within a bioreactor affecting a rapid change in the distribution of H2S and HSxe2x80x94, which has rapid effects on SRB metabolism. SRB are inhibited by H2S, and not by HSxe2x80x94, and the relative concentration of H2S and HSxe2x80x94 in solution is determined by and very sensitive to the solution pH. The relevant H2S inhibition kinetics are presented in Reis et al., (1992). The pKa of H2S (pH at which the concentrations of H2S and HSxe2x80x94 are equal) is about 7. Thus, in general, at pH somewhat lower than pH 7, more H2S is present in solution than HSxe2x80x94. At pH somewhat higher than pH 7, more HSxe2x80x94 is present in solution than H2S. While other known factors such as pH dependence of SRB activity and other inhibitors (e.g, undissociated acetate) can impact SRB metabolism, within a bioreactor where the concentration of sulfides are high, H2S inhibition can be used as the primary mechanism affecting sulfide production rate. Thus, by adjusting and controlling the pH within a pH range that does not damage the bacteria, the sulfide production rate can be quickly adjusted from between essentially zero and the maximum rate.
Increasing the inhibition of SRB acts to simultaneously slow the activity of the bacteria to essentially a standstill, thereby halting the production of sulfides, while simultaneously substantially slowing or eliminating bacterial decay and the subsequent loss of biomass concentration. By preserving biomass concentration with pH modification, when necessary, the reactor may be made to resume the production of sulfides at essentially the same rate, by returning the pH of the reactor to its prior level. Since the pH of the bioreactor may be rapidly changed through the introduction of acids and bases, the method of the present invention allows the production of sulfides in a bioreactor to be turned off and on to meet demand with no sacrifice in the rate of sulfide production. As used herein, when the pH of a bioreactor is adjusted such that the activity of the bacteria and bacterial decay are simultaneously substantially slowed or stopped, the SRBs are said to be in a state of suspended animation. It is preferred that the activity of the bacteria and bacterial decay be completely stopped, however, as will be appreciated by those having skill in the art, while the introduction of acids and bases into the bioreactor will rapidly change the pH of the bioreactor, and thereby achieve the desired state of suspended animation, slight activity of the bacteria and/or bacterial decay may persist. Thus, the present invention should be understood to encompass situations wherein some slight activity and/or decay persists. Thus, as used herein, when the rate of reduction of an oxidized sulfur compound is said to be reduced from a predetermined rate to substantially zero, the applicant intends that xe2x80x9csubstantially zeroxe2x80x9d be defined as preferably less than or equal to 10%, more preferably less than or equal to 5% and most preferably less than or equal to 1% of the predetermined rate of reduction. Also, as used herein, when a rate of decay of bacteria is said to be substantially zero, the applicant intends that xe2x80x9csubstantially zeroxe2x80x9d be defined as a rate of decay wherein preferably less than 2%, more preferably less than 0.5%, and most preferably less than 0.1% of the biomass concentration is lost in a twenty-four hour period. Finally, as used herein, when a rate of reduction of said oxidized sulfur compound is said to be substantially returned to the predetermined rate, the applicant intends that xe2x80x9csubstantially returnedxe2x80x9d be defined as preferably at least 90%, more preferably at least 95%, and most preferably at least 99% of the predetermined rate.
In a preferred embodiment, the rate of sulfide production is relative to the demand for sulfide to precipitate at least one metal from a metal-containing process waste stream. In the preferred embodiment of the invention, a first waste stream contains a quantity of metal ions requiring removal and disposal. A second process stream contains a quantity of sulfates, and can be supplemented with an organic compound to enhance growth of SRBs. The second process stream is introduced into a bioreactor and the pH is controlled from about 5.0 to about 9.0. The bioreactor can be operated in one of two distinct states: 1) at a relatively low pH, such that the SRBs are maintained in a state of suspended animation; and 2) at a relatively higher pH wherein the SRBs are actively producing sulfide for use in precipitation of metals in the first process stream.
The process of the present invention is of significant economic import to generators of metal-laden waste streams because it enables the use of a cost effective sulfide production system, biological reactors, by making the biological reactors produce sulfide xe2x80x9con demandxe2x80x9d, and therefore responsive to production schedules, waste stream generation rate, and health and safety requirements/goals. Bioreactors use non-hazardous consumables to produce the hazardous metal precipitation reagent H2S. With on-site, on-demand production of this hazardous, but effective reagent, industries can avoid storage of large quantities of hazardous material (i.e., avoid storing and using H2S gas) and tune the production of a hazardous reagent to cyclical use patterns (e.g., daily production shifts).
H2S is a preferred metal precipitation reagent because it can remove metals from aqueous streams down to residual concentrations lower than many other precipitation technologies (e.g., lime precipitation) and it does not add total dissolved solids to the waste stream. Because it only adds protons to the aqueous stream while removing the metals, use of H2S allows the possibility of water recycle/reuse within a facility.
The subject matter of the present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description taken in connection with accompanying drawings wherein like reference characters refer to like elements.